The invention relates to an arrangement for offset current compensation of a phase detector which is provided in a phase-locked loop to which an acquisition circuit is assigned which, in a switch-on phase of the phase-locked loop, supplies acquisition pulses to a loop filter arranged in the phase-locked loop before applying an input signal to the phase-locked loop so as to bring the operating frequency of a controllable oscillator in the phase-locked loop within a predetermined frequency window.
A phase-locked loop to which an acquisition circuit is assigned is known from PCT application 98-5355482 (PHN 16366). The acquisition circuit is used to bring the operating frequency of the controllable oscillator of this phase-locked loop within a frequency window after the current supply has been switched on and/or after a reset of the phase-locked loop, so as to enable the phase-locked loop to lock in on its frequency when an input signal is applied to the phase-locked loop. Particularly in the case of a narrow band construction of the phase-locked loop, this is necessary because the phase-locked loop will otherwise be unable to lock in the frequency of the input signal. This acquisition circuit thus supplies the kind of lock-in. This is realized by acquisition pulses which are fed to the loop filter and control the frequency of the controllable oscillator until its frequency is within the frequency window.
This arrangement is suitable as a lock-in in that it simplifies lock-in of the phase-locked loop on an input signal, but it is not capable of compensating offset currents of the phase detector. When the phase detector of the phase-locked loop is beset with offset currents, lock-in of the phase-locked loop on the frequency of the input signal can as yet be ensured by the acquisition circuit, but the phase-locked loop does not operate properly because it must constantly compensate the offset current of its phase detector. This in turn means that the two input signals of the phase detector are not in the desired phase position with respect to each other but in a deviating phase position which is caused by the fact that the offset current of the phase detector must be quasi compensated.